The present invention relates to a current generator, and more particularly to a generator providing, from a reference voltage Vref defined with respect to ground, a current equal to Vref/R with only a slight error, where R is a resistor value.
FIG. 1 shows a conventional circuit for such a generator. The generator comprises an operational amplifier 10 controlling the base of an NPN transistor T1, the emitter of which is connected to an inverting input of amplifier 10 and to ground G through a resistor R. The non-inverting input of amplifier 10 is provided with a reference voltage Vref with respect to ground. This reference voltage may be provided, for example, by a conventional reference voltage generator of the "Band-Gap" type, the temperature of which is stabilized. The collector of transistor T1 is connected to a current output terminal S which provides the reference current to a node of the circuit.
With this configuration, the voltage across resistor R is held to Vref, and the emitter current of transistor T1 is therefore Vref/R. The collector current Ic of transistor T1 (output current) is therefore approximately: EQU Ic.apprxeq.(1-1/.beta.) Vref/R, (1)
where .beta. designates the current gain of transistor T1. In the presently preferred embodiment, this is typically about 90; but of course this specific value is supplied merely to illustrate the best mode, and is not at all necessary for practising the invention. The generator provides a current proportional to Vref which exhibits a temperature accuracy of approximately 2% over the range from -55.degree. to +125.degree. C. The inaccuracy essentially originates from the term 1/.beta.. By using a MOS transistor instead of the bipolar transistor T1, this term can be made essentially zero, which improves accuracy.
However, the implementation of such a current source requires the use of an operational amplifier. Such an operational amplifier typically includes a large number of components (about 12 transistors), and must also be connected to a compensating capacitor (not shown), since the operational amplifier operates in a closed loop mode with a unit gain. In an integrated circuit, such a current source therefore occupies a large area of silicon.
Moreover, the voltage between terminal S and ground has to remain higher than a minimum value equal to Vref+Vcesat, where Vcesat designates the emitter-collector voltage of a bipolar transistor in the saturation state. This minimum value is generally higher when a MOS transistor is used instead of transistor T1. Thus, it is not possible to properly provide current to a node having a variable voltage which may become lower than Vref+Vcesat.
The disclosed innovations advantageously provide a precise current generator which, when integrated, occupies a small silicon surface.
The disclosed innovations advantageously provide a precise current generator capable of providing a precise current to a node, the voltage of which varies within a large range.
The presently preferred embodiment provides a current generator comprising a first bipolar transistor, the base of which is connected to a reference voltage and the emitter to a first supply voltage through a first resistor; a first current mirror, the input of which is connected to the collector of the first transistor and the output is connected to the control electrode of a second transistor; a third bipolar transistor, the collector of which is connected to the output of the first current mirror, the base of which is connected to a main electrode of the second transistor and to a terminal of a second resistor, the other terminal of which is connected to the emitter of the third transistor; and a second current mirror, the input of which is connected to the emitter of the third transistor and the output provides the desired current.
According to an embodiment of the invention, the second current mirror comprises a fourth transistor, the collector of which forms the mirror input and the emitter of which is connected to the first supply voltage; at least a fifth transistor parallel-connected with the base and the emitter of the fourth transistor and the collector of which provides the desired current; and a sixth transistor, the emitter of which is connected to the base of the fourth transistor and the base is connected either to the base or to the emitter of the third transistor.
According to an embodiment of the invention, the second transistor can be either a MOS transistor, a bipolar transistor or a Darlington transistor pair.
According to an embodiment of the invention, the first current mirror is a Wilson-type mirror.
An advantage of the invention is that it is particularly adapted for manufacturing current generators with multiple outputs.
The foregoing and other objects, features, aspects and advantages of the invention will become apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.